Imagine this: you have opened several terminal windows which cover most of your desktop, and a Firefox browser window. In Firefox you have an image which contains some data that you must type to one of the terminal windows. Since the data is in an image (not as a text), you can't use cut & paste to transfer it from Firefox to terminal.

In GNOME2 (and Windows), you can interleave the Firefox window and the terminal window so that the terminal window is on top of the Firefox window. You can place the terminal window so that it does not cover that part of the Firefox window which contains the data that you must type to the terminal window.

But in GNOME3, if I activate the terminal window, all open terminal windows are shown on top the Firefox window. In order to see the Firefox window again, I have to minimize or move those terminal windows which cover the part which I want to see while typing into the one terminal window. I think this is clumsy. Is there any way to switch back to the GNOME2 behavior which allows interleaving windows from different applications?

I fail to see the problem here. If I have Firefox active and two different terminals open behind it, I can go to Activities and select one of the terminals and it comes active on top of Firefox.

You can also do this neat trick:
Grab your firefox window and drag it to the left of your screen, it now fills 50% of your screen. Then grab your terminal window and drag it to the right of your screen. You should now have firefox and termianl side by side.